Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Sat, 21 Sep 2002 19:11:26 -0400
Message-ID: <redirect-1740986@logan.com>
From: LancairJim@aol.com
Subject: Re:Fuel Tank Check Valve LNC2
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There is something about this business of adding check valves in the fuel 
tank vent lines that I don't understand. Hopefully, someone on the LML can 
explain it.

The primary purpose of the fuel tank vent is to allow air to flow into the 
tank as fuel is withdrawn. However, the vent also has the secondary, but no 
less important task, of equalizing pressure inside the tank with ambient 
conditions. For example, if the plane is refueled at an airport which is at 
sea level, the air pressure within the tank will be at equilibrium with the 
atmosphere at roughly 14.7 psia. If the plane then departs and climbs to say 
10,000 ft, atmospheric pressure will have dropped to roughly 10.1 psia. If 
there is a check valve in the vent line, air inside the tank cannot flow out 
the vent during the climb to equalize the pressure. Under these conditions, 
the pressure inside the tank will still be 14.7 psia and the pilot will be 
performing an inadvertent pressure test on the plane's fuel tanks with a 
differential pressure of 4.6 psid (i.e., 14.7 psia inside the tank minus 10.1 
psia outside).  I have read of builders rupturing their tanks at 
substantially lower differential pressures than this during leak testing. At 
20,000 ft the theoretical differential pressure is about 7.9 psid. Implicit 
in this analysis are several assumptions:

1) A significant quantity of fuel is not withdrawn from the tank during the 
climb.
2) The fuel cap doesn't leak or depart the plane.
3) The fuel tank doesn't leak.
4) The check valve doesn't leak.
5) Temperature effects are presumed to be negligible.

Is there something wrong with this analysis? What am I missing? Does everyone 
who installs a check valve in their vent line just assume that the check 
valve or gas cap will leak a sufficient amount to prevent overpressurization 
of the tank? Seems like a gross rationalization to me. If you are wrong, you 
could lose a helluva lot more than a few pints of petrol.

Until someone can explain to my satisfaction how overpressurization is not an 
issue, I think I will continue to build my LNC2 without check valves in the 
vent lines and put up with the occasional loss of fuel.

Jim McKibbin

PS - A float valve makes a lot more sense to me than a check valve from a 
functional standpoint but I haven't a clue where you might get a suitable one 
or how to install it.